1. Technical Field
This invention relates to a printed circuit board in which the upper wiring is connected with the lower wiring through the conductor pattern on the bottom and internal wall of a blind hole, and a process for the manufacture thereof.
2. Description of the Prior Art
In the manufacture of printed circuit boards, attempts to reduce the land to as little as possible have been made in order to increase their wiring density. A typical example is the landless substrate, in which wiring density is increased by eliminating the land when connecting wiring using plated through hole (PTH).
Such a landless substrate has, as FIG. 3 shows for example, a PTH with a plated layer 5 on the internal wall of a through hole 2 formed in the substrate 1, to connect the connection line 3 on the top with the connection line 4 on the bottom. Such a landless substrate is formed by applying a resin layer in the internal wall of the through hole, plating the resin layer with solder, and etching off the solder plating, leaving a positive photoresist on the vertical wall (internal wall) of the solder plating.
However, in such a landless substrate it is impossible to connect the inner lines 6, as shown in FIG. 3, with the vertical wall of the solder plating. On the other hand, it is possible to connect the inner lines 6 with the vertical wall of the solder plating without forming a resin layer on the internal wall of the through hole. FIG. 4 shows the connection thus formed. (A) is a top view and (B) is a sectional view. As these diagrams show, when the inner lines 6 are thus connected with a plated layer 7, the connection area is only the width .times. thickness of the inner lines 6. Therefore, the connection of the inner lines 6 with the plated layer 7 lacks reliability.
As FIG. 5 shows, landless via connection is also feasible by forming a drill hole to the middle of the thickness direction of the substrate 1 so as to cut the inner lines 6, and forming a plated layer 8 by etching using the positive photoresist on the internal wall of the drill hole as described above. In this case, however, connection reliability cannot be obtained.
On the other hand, a structure to connect upper wiring, outer layer wiring, for example, with inner layer wiring, using a blind hole which reaches the inner wiring, i.e., lower layer wiring, has been known. The process for manufacturing such a structure is shown in FIG. 6. As FIG. 6 shows, a photosensitive resin layer 13 is applied to the substrate 11 so as to cover wiring 12 which will become the inner wiring formed on the substrate 11 (FIG. 6(a)), the photosensitive layer 13 is irradiated through a mask pattern 14 having a mask 14a the size of the blind hole so as to expose parts other than the mask pattern (FIG. 6(b)), and is developed to form a blind hole 15 extending to the inner wiring 12 (FIG. 6(c)). Then, a conductor layer 16 is formed on the bottom and the internal wall of the blind hole 15 and the surface of the substrate 11 (FIG. 6(d)), a dry film photoresist 17 is applied so as to cover the blind hole 15, and the photoresist 17 is exposed through a mask pattern 18 having a mask 18a of a size slightly larger than the opening of the blind hole 15 covering the bottom, even if a certain misalignment is considered, to expose parts of the photoresist 17 other than the mask 18a (FIG. 6(e)). Then, a resist pattern to cover the opening of the blind hole 15 is formed by developing (FIG. 6(f)). Then, a conductor pattern 16a having a land around the opening of the blind hole 15 and being connected to the inner wiring 12 are created by etching. A structure to connect the inner wiring 12 with surface wiring through the conductor pattern 16a is formed by forming the surface wiring (not shown) on the substrate 11 continuous to the land of the conductor pattern 16a using a known method.
Although reliability of connection with the inner wiring is obtained by the structure as shown in FIG. 6, the requirement of increasing wiring density cannot be met because of the presence of a land around the opening of the blind hole.
Therefore, an object of this invention is to provide a printed circuit board to connect the upper layer wiring with lower layer wiring, and to achieve a high wiring density, as well as a process for manufacturing such a printed circuit board.
Another object of this invention is to provide a printed circuit board in which the inner wiring are reliably connected and to realize a high wiring density.